Studies on age changes in physiologic systems have related performance in exercise (peak V02), and kidney function (creatinine clearance) to parameters of bone mineral homeostasis (ionized calcium, phosphorous, vitamin D metabolites, and PTH) and to measures of bone mineral density (BMD) (single and dual photon absorptiometry). There were no independent effects of level of performance on exercise testing on either the biochemical parameters or on the level of BMD in the hip, spine or forearm, beyond that due to age. This was demonstrated by multiple regression analysis and by comparing mean values for the parameters and BMD in two parts of the normal population, the third of the population that had the best age-adjusted performance on peak V02 and the third of the population with the worst age-adjusted performance. Similar analyses for performance on kidney function showed the same results. Any biochemical or BMD variable related to creatinine clearance was also related to age. The effect of kidney function was not demonstrable once the age effect was taken into account. Cross-sectional and longitudinal analyses agree that grip strength declines at an accelerating rate after age 40. With advancing age, grip strength deteriorates more than is predicted by the age-related decline in muscle mass.